The present invention relates to flight control systems for rotary wing aircraft, and more particularly to such flight control systems which provide effective yaw control.
In a helicopter, and in particular in an attack helicopter, the responsive of the helicopter about its yaw axis is crucial in combat situations. The aircraft must be capable of quickly moving about its yaw axis to bring its weapons to bear on a target. Therefore the flight control system has to be capable of responding with a tail rotor command which provides such responsiveness when a yaw input is received from the pilot.
One type of yaw maneuver requires the helicopter to travel forward along a first axis then rapidly yaw to point off the first axis while maintaining forward motion along the first axis. However, upon performance of such a maneuver, known flight control systems must be assisted by pilot input. The pilot input commonly requires the pilot to xe2x80x9cstirxe2x80x9d the cyclic during the yaw maneuver. Although effective, pilot workload is accordingly increased. A similar situation occurs when performing a yaw maneuver while hovering in a high wind condition.
Accordingly, it is desirable to provide a flight control system which automatically provides compensatory trim during yaw maneuvers to reduce pilot workload which thereby leads to a more capable attack helicopter.
The flight control system according to the present invention provides a trim augmentation algorithm which advantageously provides two primary functions: 1.) direct augmentation of the rotor trim via on-axis stick input; and 2.) automatic cross-axis trim transfer as a function of the commanded yaw rate.
In a first function of the present invention, direct trim augmentation is active in each axis (pitch and roll) when the pilot stick input is in the same direction as the rate error feedback signal. In other words, when the aircraft response is lagging the pilot""s commanded maneuver state. When the direct augmentation logic in pitch and/or roll is true the stick command is multiplied by the gain, limited, and summed with the below described cross-axis trim transfer function at a summing junction to direct the actual vehicle response toward said pilot commanded rate signal and improve the performance of the model following control laws.
When a helicopter turns in winds, the total trim vector (sum of pitch and roll control vectors) remains nearly constant with respect to the direction of the wind. In other words, the trim vector stays fixed, while the aircraft rotates (yaws). In high winds there may be significant differences between the magnitudes of the pitch and roll trim positions (to cancel wind induced forces on the vehicle/rotor). These trim positions change significantly when the aircraft turns in the wind.
In a second function of the present invention, a cross-axis trim transfer function of the trim augmentation algorithm uses the commanded yaw rate to anticipate the trim change when the aircraft is rotating in winds. The commanded yaw rate times the scaled trim error yields the rate of change of the trim in the opposite axis (pitch to roll/roll to pitch). In other words, the relationship of the rotor trim vector to the wind is maintained independent of the rotation of the helicopter body therebelow. Thus, when a yaw maneuver is performed, the cross-axis trim transfer function provides for the automatic precession of the rotor control vector (trim state) relative to the aircraft body such that the pilot work load is reduced. In other words, the cross-axis trim transfer function automatically xe2x80x9cstirsxe2x80x9d the cyclic as the aircraft rotates about the yaw axis. The present invention therefore reduces the amount of compensatory pilot trim which must be input during yaw maneuvers which thereby leads to a more capable attack helicopter.